Last Updated on June 3, 2023 by Dave Farquhar
It’s a rule, right? No project is complete without three unplanned trips to the hardware store. Needing screws for an electrical box can absolutely chew up one of those trips. But I can at least make it easier. In this blog post, I’ll explain electrical box screw size, or sizes, because in the United States at least, it’s almost certainly one of three sizes. There’s no need to fumble or guess at what size screws to use in an electrical box.
The most common electrical box screw size is 6-32 machine screw. However, for many applications, 6-32 is too light duty, so we use an 8-32 or even a 10-32 instead. The thread pitch, however, is always 32.
Table of Contents
- Electrical box screw size for outlets and switches
- What size are outlet screws?
- Extra long screws for outlets
- Light fixture screws
- Electrical box screw size for junction boxes
- Screw size for electrical box ground screw
- Being prepared
Electrical box screw size for outlets and switches
The most common electrical box screw size is a 6-32 machine screw, and you can take advantage of this to try to save yourself a trip, or buy yourself some time to reduce the number of trips. The most common size inside an electrical box is 6-32, and that’s also the same size screw that you use to hold cover plates on outlets and switches. So in a pinch, you can borrow a 6-32 from a light switch to verify the size. Use an oval-head screw for cover plates. Inside the box, use round-head screws to get the best electrical contact and prevent arcing.
Just don’t try to substitute a #6 wood screw or sheet metal screw in an electrical box. The diameter is the same but the thread is different, so it will cross threads and damage them.
If you find a 6-32 screw is just slightly too small in the electrical box you’re working on, then you need an 8-32. If the 6-32 is noticeably too small, you need a 10-32.
You can also use a drill bit to gauge the appropriate electrical box screw size. A 6-32 screw is .138 inches in diameter. If a 1/8-inch drill bit fits the hole with just a little room to spare, you need a 6-32.
What size are outlet screws?
Outlet screws are 6-32 machine screws. The length of outlet screws can vary. I find 1/2-inch to 1-inch length screws almost always fit, except in the case of GFCI outlets. It’s safest to go with 5/16-inch for GFCI. GFCIs often go into tile walls and the shorter screws accommodate the tile. When installing GFCIs, I frequently end up substituting shorter screws.
When substituting screws or replacing an outlet, I always salvage the screws from the old outlet or unused screws from the package and put them in my small parts organizer. That way I’m more likely to always have plenty of suitable outlet screws on hand when I need to replace a missing screw or substitute a more appropriate size.
Extra long screws for outlets
There have been a few times, such as when dealing with electrical outlet boxes recessed deep into walls, that I’ve needed longer screws than the usual 1/2-inch to 1-inch range. I’ve sometimes had to use extension boxes and extra long screws to deal with that situation. For extra long screws for outlets, simply buy 6-32 machine screws in whatever length gets you the reach you need.
Keep in mind the wall itself has around four inches of clearance if there’s nothing behind the outlet box. Electrical outlet boxes are sometimes 1 1/2 inches deep to accommodate being installed back-to-back in a wall. Add half an inch for the surface of the wall, and you have room for a 2-inch screw if you’re concerned about clearance.
So if you need extra long screws for outlets, get 6-32 machine screws somewhere between 2 inches to 4 inches as appropriate. I keep a supply of 2-inch 6-32 machine screws in my parts organizer to serve as extra long electrical box screws when I need them.
Light fixture screws
Light fixtures have to support weight, so light fixture electrical boxes have threads for 8-32 machine screws. That means light fixture screws are 8-32, heavier than outlet screws, and the length always varies.
I find the screws that come with light fixtures are usually shorter than I’d like. That makes it fiddly to line up the fixture when finishing up installation. For light fixture screws, I like to use 8-32 screws of 2 inches in length or more. That way I can leave lots of slack in each screw. The slack lets me line up one side of the fixture, then tilt and swivel the fixture as much as I need to line up the other. Once the fixture is lined up, I can tighten one screw deep enough to lock it in, then drive in the other. I find using long light fixture screws helps me install light fixtures much more easily.
I’m sure someone who does this every day develops a feel for how the fixture lines up with the electrical box and can get it done with whatever screws came with the light fixture. A DIY weekend warrior will struggle less with a bit more wiggle room.
I keep a supply of 2-inch 8-32 machine screws in my parts organizer for replacement light fixture screws. Whatever screws came with the light fixture go into my organizer for other uses. Having that assortment of screws that fit electrical boxes has saved me countless return trips to the hardware store.
Electrical box screw size for junction boxes
Junction boxes usually use a heavier duty screw, an 8-32. Junction boxes often hold heavier light fixtures, so they need a screw that can hold a bit more weight. 8-32 is a very common size, at least. An 8-32 is .164 inches in diameter, which is a shade over 5/32. A 5/32 bit will not fit in place of a 6-32.
In this case, the length depends almost entirely on application. As I mentioned previously, when hanging a light fixture, I like to replace the screws that came with the fixture with longer ones. I find having some extra slack on the screws makes it easier to line the holes on the fixture up with the screws. If I have very little slack I have to get both of them right, but if I have a lot of slack, I can get one, then find the other. Then I tighten the screws.
And finally, screw size for electrical box ground screw
Ground screws for electrical work are larger still, generally, 10-32, and generally 5/16″ long, but you can usually thread a longer screw in. By convention they are usually green, to indicate to everyone what they are. But in a pinch, a regular bare metal 10-32 screw will work for ground. Steel doesn’t conduct as well as copper, but making it thicker decreases resistance. A 3/16″ diameter of bare steel conducts electricity pretty well. Hex heads are common on ground screws in electrical boxes because it gives you multiple options for driving it in, and hex heads rarely strip. You can substitute a round head if you wish. Just don’t use a flat or oval head, as those types of heads won’t make very good contact with the wire.
I keep a small parts organizer containing various lengths of 6-32 and 8-32 screws, since both of them are so common. It’s saved me a lot of hassle since I bought that organizer and populated it. This assortment of 8-32s is a good buy for getting started. If you don’t do a lot of electrical work it can be overkill, but I find a lot of other things I’m interested in also use 6-32 and 8-32 screws extensively. Even vintage Erector sets used 8-32s and 6-32s. Really.
Whenever I replace the screws that come with boxes or fixtures with a longer screw, I put the screws that came in the package in my organizer, sorted by size. I’ll end up needing those for some other purpose eventually.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.